Write What You Know...But Be Warned
There's been a lot of debate about this lately, with one faction claiming that the idea of NOT writing what you know panics people for two reasons. First, writers have been encouraged, explicitly or implicitly, for as long as they can remember, to write what they know, so the prospect of abandoning that approach now is disorienting. Second, a great many of us know an awful lot.
I'm not here to tell you what to do - or not do - but to tell you what I do and some possible consequences. I write what I know. Most of my characters, their characteristics, their motivations, are lifted (in part) from people I know or have met in the course of my travels. I feel I can paint a more realistic story by using these core character elements and molding certain features where I need to.
But you should be warned, if you do like I do and commonly use friends and family as first draft readers, then it is likely that you could accidentally piss one of them off. This particular reader pool can and will easily pick up on traits you've borrowed from a shared acquaintance and try and match them to their respective owners - which sometimes are themselves. When that happens accidental transference can take place. I've experienced it. You could be using a moral stance, or an ethical quandary, one that is totally imaginary, and a reader could see it as your interpretation -- mistakenly -- of a belief of theirs and feelings can get hurt. Most of the time its out of your control because the people involved rarely tell you how they feel. Heck -- even when you are describing them in the most honest way you can, that person might still not like what he see's in your mirror.
Just some food for thought. If one of those inner-reading circle people stops returning your calls suddenly, maybe you should take a closer look at your manuscript. :)